Base station, mobile station, radio communication system, and communication control method

ABSTRACT

One of sequence groups each specifying reference signal sequences for respective radio resource bandwidths is assigned to a base station and a different one of the sequence groups is assigned to a neighboring cell. The base station includes a scheduler configured to allocate radio resources; a reporting unit configured to report the allocated radio resources and a cyclic shift amount to a mobile station; and a demodulating unit configured to demodulate a signal received from the mobile station based on one of the reference signal sequences corresponding to one of the radio resource bandwidths and the cyclic shift amount. Cell reuse is applied to the reference signal sequences to be transmitted using one resource unit, and sequence hopping where different ones of the reference signal sequences are assigned to consecutive subframes is applied to the reference signal sequences to be transmitted using a bandwidth greater than one resource unit.

TECHNICAL FIELD

The present invention generally relates to a radio communication system.More particularly, the present invention relates to a base station, amobile station, a radio communication system, and a communicationcontrol method.

BACKGROUND ART

A successor communication system to W-CDMA and HSDPA, i.e., Long TermEvolution (LTE), is currently being discussed by 3GPP, a standardizationgroup for W-CDMA. In LTE, orthogonal frequency division multiplexing(OFDM) is to be used as a downlink radio access method andsingle-carrier frequency division multiple access (SC-FDMA) is to beused as an uplink radio access method (see, for example, 3GPP TR 25.814(V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006).

In OFDM, a frequency band is divided into multiple narrow frequencybands (subcarriers) and data are transmitted on the subcarriers. Thesubcarriers are densely arranged along the frequency axis such that theypartly overlap each other but do not interfere with each other. Thisapproach enables high-speed transmission and improves frequencyefficiency.

In SC-FDMA, a frequency band is divided into multiple frequency bandsand the frequency bands are allocated to different terminals fortransmission in order to reduce interference between the terminals.Also, SC-FDMA reduces variation of the transmission power and thereforemakes it possible to reduce power consumption of terminals and toachieve wide coverage.

A reference signal for uplink in E-UTRA indicates a pilot channel thatis used for purposes such as synchronization, channel estimation forcoherent detection, and measurement of received SINR in transmissionpower control. The reference signal is a transmission signal known tothe receiving end, i.e., the base station, and is embedded at intervalsin each subframe.

In W-CDMA, a user-specific PN sequence, more precisely, a signalsequence obtained by multiplying a long-cycle Gold sequence and anorthogonal sequence, is used as the reference signal (pilot channel).Since the PN sequence is long, it is possible to generate many differentPN sequences. However, since the correlation properties of PN sequencesare poor, the accuracy of channel estimation may become low. In otherwords, the interference between a pilot channel of a user and a pilotchannel of another user may become high. Also, in a multipathenvironment, the autocorrelation between a pilot channel sequence andits delayed wave becomes high. In W-CDMA, simple reception processingsuch as RAKE reception is employed. Meanwhile, an E-UTRA system isdesigned to suppress the multipath interference based on highly-accuratechannel estimation using, for example, an equalizer. For this reason, inE-UTRA, a constant amplitude and zero auto-correlation (CAZAC) sequenceis used instead of a user-specific PN sequence.

The CAZAC sequence has excellent autocorrelation properties andcross-correlation properties and therefore enables highly-accuratechannel estimation. In other words, compared with the PN sequence, theCAZAC sequence makes it possible to greatly improve the demodulationaccuracy. With the CAZAC sequence, the variation in the amplitude of asignal is small both in the frequency domain and the time domain, i.e.,the amplitude of the signal becomes comparatively flat. Meanwhile, withthe PN sequence, the variation in the amplitude of a signal is large inthe frequency domain. Thus, using the CAZAC sequence makes it possibleto accurately perform channel estimation for each frequency using anequalizer. Also, since the autocorrelation of a transmitted CAZACsequence becomes zero, it is possible to reduce the influence ofmultipath interference.

Still, the CAZAC sequence has problems as described below.

-   -   The number of sequences is small.

Since it is not possible to assign unique CAZAC sequences to respectiveusers, it is necessary to repeatedly or cyclically assign a limitednumber of CAZAC sequences to multiple cells (hereafter, this is called“cell reuse”). The number of sequences becomes particularly small whenthe transmission band in SC-FDMA is narrow. In other words, when thetransmission band in SC-FDMA is narrow, the symbol rate becomes low andthe CAZAC sequence length decreases. In E-UTRA, a reference signal istime-division-multiplexed. Therefore, the symbol rate becomes low andthe sequence length decreases when the transmission band is narrow. Thenumber of sequences corresponds to the sequence length. For example,when the sequence length is 12 symbols in a transmission band of 180kHz, it is not possible to assign user-specific sequences and thereforeit is necessary to repeatedly or cyclically assign 12 sequences tomultiple cells (may be greater than 12) such that the same sequence isnot assigned to neighboring cells.

-   -   Cross-correlation between CAZAC sequences with different lengths        varies rather greatly depending on the combination of the CAZAC        sequences. When the cross-correlation is high, the accuracy of        channel estimation is reduced.

Next, SC-FDMA used as an uplink radio access method in E-UTRA isdescribed with reference to FIG. 1. In SC-FDMA, a system frequency bandis divided into multiple resource blocks each of which includes one ormore subcarriers. Each user device (user equipment: UE) is allocated oneor more resource blocks. In frequency scheduling, to improve thetransmission efficiency or the throughput of the entire system, resourceblocks are allocated preferentially to user devices with good channelconditions according to received signal quality or channel qualityindicators (CQIs) measured and reported based on downlink pilot channelsfor the respective resource blocks by the user devices. Frequencyhopping where allocation of frequency blocks is varied according to afrequency hopping pattern may also be employed.

In FIG. 1, time and frequency resources allocated to different users arerepresented by different hatchings. For example, a relatively widefrequency band is allocated to UE2 in the first subframe, but arelatively narrow frequency band is allocated to UE2 in the nextsubframe. Different frequency bands are allocated to the respectiveusers such that the frequency bands do not overlap.

In SC-FDMA, different time and frequency resources are allocated torespective users in a cell for transmission to achieve orthogonalitybetween the users in the cell. Here, the minimum unit of the time andfrequency resources is called a resource unit (RU). In SC-FDMA, aconsecutive frequency band is allocated to each user to achievesingle-carrier transmission with a low peak-to-average power ratio(PAPR). Allocation of the time and frequency resources in SC-FDMA isdetermined by a scheduler of the base station based on propagationconditions of respective users and the quality of service (QoS) of datato be transmitted. The QoS includes a data rate, a desired error rate,and a delay. Thus, in SC-FDMA, the system throughput is improved byallocating time and frequency resources providing good propagationconditions to respective users.

Respective base stations separately determine allocation of time andfrequency resources. Therefore, a frequency band allocated in a cell mayoverlap a frequency band allocated in a neighboring cell. If frequencybands allocated in neighboring cells partly overlap, signals interferewith each other and their quality is reduced.

Next, a reference signal in uplink SC-FDMA is described with referenceto FIG. 2. FIG. 2 shows an example of a subframe structure.

The packet length of a TTI called a subframe is 1 ms. One subframeincludes 14 blocks to be submitted to FFT. Two of the 14 blocks are usedfor transmission of a reference signal and the remaining 12 blocks areused for transmission of data.

The reference signal is time-division-multiplexed with a data channel.The transmission bandwidth is dynamically changed according to theresults of frequency scheduling by the base station. When thetransmission bandwidth decreases, the symbol rate decreases and thesequence length of a reference signal to be transmitted in a fixed timeperiod decreases. When the transmission bandwidth increases, the symbolrate increases and the sequence length of a reference signal to betransmitted in a fixed time period increases. When a reference signal isto be transmitted using a narrow band, for example, a 180 kHz band thatequals one resource unit or 12 subcarriers, the number of symbolsbecomes 12. In this case, both the sequence length and the number ofsequences become about 12. When a reference signal is to be transmittedusing a wide band, for example, a 4.5 MHz band that equals 25 resourceunits or 300 subcarriers, the number of symbols becomes 300. In thiscase, both the sequence length and the number of sequences become about300.

Meanwhile, orthogonalization of multiple reference signals by usingcyclically-shifted CAZAC sequences is proposed. As shown in FIG. 3, whencyclically-shifted CAZAC sequences are used and all multipaths arewithin the amount of cyclic shift (cyclic shift amount), it is possibleto orthogonalize reference signals of different users and antennas. Evenwhen different users transmit the same sequence at the same timing usingthe same frequency band, it is possible to orthogonalize the users bycyclically shifting the sequence.

It is also proposed to orthogonalize two reference signals by employingorthogonal covering. In orthogonal covering, as shown in FIG. 4, users 1and 2 may use different CAZAC sequences and different cyclic shiftamounts as long as the same CAZAC sequence and the same cyclic shiftamount are used for two reference signals in a subframe. With thisapproach, after the two reference signals are despread, the users becomeorthogonal with each other.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, the above background art technologies have problems asdescribed below.

When the same frequency band with the same bandwidth is used in the basestation's own cell and an interfering cell, the cross-correlation(interference) between reference signals using CAZAC sequences becomesvery low and therefore the communication quality is improved.

However, in SC-FDMA, since the frequency band and the bandwidthallocated to a user change from time to time according to schedulingresults, it is rare that the same frequency band with the same bandwidthis allocated to both the user and an interfering station. In otherwords, in most cases, even if the bandwidths are the same, differentfrequency bands are allocated to the user and the interfering station.In such a case, the interference or correlation between a certaincombination of CAZAC sequences may become high and the communicationquality may be reduced. As described above, since the number of CAZACsequences usable for reference signals is small, CAZAC sequences areassigned cell by cell. If one CAZAC sequence is always used for abandwidth in a cell, interference may become continuously high in allframes and the communication quality may be reduced greatly depending onthe combination of the bandwidth used in the cell and a bandwidth usedin an interfering cell. In W-CDMA, the interval between referencesignals is very long. Therefore, even if high interference is observedin a frame, it is expected that the interference decreases in asucceeding frame.

To prevent interference from becoming continuously high intemporally-consecutive frames, one of the following two methods may beemployed: sequence hopping where different CAZAC sequences are used inthe consecutive frames; and cyclic-shift hopping where the amount ofcyclically shifting the CAZAC sequence in the time direction is changed(hopped) for each frame.

With the cyclic-shift hopping, since the number of cyclic shift amountsis limited to about six when the CP length and the delay spread aretaken into account, the effect of interference randomization is limited.With the sequence hopping, a greater effect of interferencerandomization can be expected particularly when a wide frequency band isused, i.e., when a large number of sequences are available. However,when a narrow frequency band, for example, one resource unit (RU), isused, only about 12 sequences can be generated. Therefore, even if theCAZAC sequences are randomly changed (hopped), the same CAZAC sequencemay be used about once in 12 frames in neighboring cells and as aresult, a packet error may be caused.

One object of the present invention is to solve or reduce one or more ofthe above problems and to provide a base station, a mobile station, aradio communication system, and a communication control method that makeit possible to flexibly reuse CAZAC reference signal sequences formultiple cells in an E-UTRA system and thereby to reduce the influenceof characteristic degradation caused by cross-correlation.

Means for Solving the Problems

An aspect of the present invention provides a base station forcommunicating with a mobile station transmitting an uplink signalaccording to a single-carrier scheme. One of sequence groups eachspecifying reference signal sequences for respective radio resourcebandwidths is assigned to the base station, a different one of thesequence groups is assigned to a neighboring cell, and the mobilestation transmits the uplink signal including one of the referencesignal sequences specified by one of the sequence groups assigned to thebase station. The base station includes a scheduler configured toallocate radio resources such that one or more resource units areallocated to the mobile station for communication; a reporting unitconfigured to report the allocated radio resources and a cyclic shiftamount to the mobile station; and a demodulating unit configured todemodulate the uplink signal received from the mobile station based onone of the reference signal sequences corresponding to one of the radioresource bandwidths and the cyclic shift amount. Cell reuse is appliedto the reference signal sequences to be transmitted using one resourceunit, and sequence hopping where different ones of the reference signalsequences are assigned to consecutive subframes is applied to thereference signal sequences to be transmitted using a bandwidth greaterthan one resource unit.

Another aspect of the present invention provides a mobile stationtransmitting an uplink signal according to a single-carrier scheme. Themobile station includes a storage unit configured to store sequencegroups each specifying reference signal sequences for respective radioresource bandwidths; a transmitting unit configured to determine one ofthe reference signal sequences corresponding to one of the radioresource bandwidths based on radio resources allocated by a basestation, to shift the determined one of the reference signal sequencesby a cyclic shift amount assigned by the base station, and to transmitthe uplink signal including the shifted one of the reference signalsequences. Cell reuse is applied to the reference signal sequences to betransmitted using one resource unit, and sequence hopping wheredifferent ones of the reference signal sequences are assigned toconsecutive subframes is applied to the reference signal sequences to betransmitted using a bandwidth greater than one resource unit.

Another aspect of the present invention provides a radio communicationsystem including a mobile station configured to transmit an uplinksignal according to a single-carrier scheme; and a base stationconfigured to communicate with the mobile station. One of sequencegroups each specifying reference signal sequences for respective radioresource bandwidths is assigned to the base station, and a different oneof the sequence groups is assigned to a neighboring cell. The mobilestation is configured to transmit the uplink signal including one of thereference signal sequences specified by one of the sequence groupsassigned to the base station. The base station includes a schedulerconfigured to allocate radio resources such that one or more resourceunits are allocated to the mobile station for communication, a reportingunit configured to report the allocated radio resources and a cyclicshift amount to the mobile station, and a demodulating unit configuredto demodulate the uplink signal received from the mobile station basedon one of the reference signal sequences corresponding to one of theradio resource bandwidths and the cyclic shift amount. The mobilestation includes a storage unit configured to store the sequence groups;and a transmitting unit configured to determine one of the referencesignal sequences corresponding to one of the radio resource bandwidthsbased on the radio resources allocated by the base station, to shift thedetermined one of the reference signal sequences by the cyclic shiftamount reported by the base station, and to transmit the uplink signalincluding the shifted one of the reference signal sequences. Cell reuseis applied to the reference signal sequences to be transmitted using oneresource unit, and sequence hopping where different ones of thereference signal sequences are assigned to consecutive subframes isapplied to the reference signal sequences to be transmitted using abandwidth greater than one resource unit.

Still another aspect of the present invention provides a communicationcontrol method used in a radio communication system including a mobilestation transmitting an uplink signal according to a single-carrierscheme and a base station communicating with the mobile station. One ofsequence groups each specifying reference signal sequences forrespective radio resource bandwidths is assigned to the base station anda different one of the sequence groups is assigned to a neighboringcell. The method including a radio resource allocation step, performedby the base station, of allocating radio resources such that one or moreresource units are allocated to the mobile station for communication; areporting step, performed by the base station, of reporting theallocated radio resources and a cyclic shift amount to the mobilestation; a transmitting step, performed by the mobile station, oftransmitting the uplink signal based on the radio resources and thecyclic shift amount reported by the base station; and a demodulatingstep, performed by the base station, of demodulating the uplink signalreceived from the mobile station based on one of the reference signalsequences corresponding to one of the radio resource bandwidths and thecyclic shift amount. Cell reuse is applied to the reference signalsequences to be transmitted using one resource unit, and sequencehopping where different ones of the reference signal sequences areassigned to consecutive subframes is applied to the reference signalsequences to be transmitted using a bandwidth greater than one resourceunit.

ADVANTAGEOUS EFFECT OF THE INVENTION

An aspect of the present invention provides a base station, a mobilestation, a radio communication system, and a communication controlmethod that make it possible to flexibly reuse CAZAC reference signalsequences for multiple cells in an E-UTRA system and thereby to reducethe influence of characteristic degradation caused by cross-correlation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating single-carrier FDMA;

FIG. 2 is a drawing illustrating reference signals used insingle-carrier FDMA;

FIG. 3 is a drawing illustrating reference signals used insingle-carrier FDMA;

FIG. 4 is a drawing illustrating reference signals used insingle-carrier FDMA;

FIG. 5 is a drawing illustrating a radio communication system accordingto an embodiment of the present invention;

FIG. 6 is a table showing exemplary assignment of reference signalsequences according to an embodiment of the present invention;

FIG. 7 is a partial block diagram of a base station according to anembodiment of the present invention;

FIG. 8 is a partial block diagram of a mobile station according to anembodiment of the present invention;

FIG. 9 is a drawing illustrating an exemplary method of assigningreference signal sequences according to an embodiment of the presentinvention;

FIG. 10 is a drawing illustrating selective use of orthogonalization andinterference randomization for base stations according to an embodimentof the present invention; and

FIG. 11 is a flowchart showing a process in a radio communication systemaccording to an embodiment of the present invention.

EXPLANATION OF REFERENCES

-   -   50 Cell    -   100 ₁, 100 ₂, 100 ₃, 100 _(n) Mobile station    -   102 OFDM signal demodulation unit    -   104 Uplink-scheduling-grant-signal demodulation/decoding unit    -   106 Broadcast-channel demodulation/decoding unit    -   108 Other-control-and-data-signals demodulation/decoding unit    -   110 Radio-frame-number-and-subframe-number counter    -   112 Cyclic-shift-amount determining unit    -   114 Memory for storing RS sequence numbers associated with        bandwidths in RS sequence groups    -   116 Demodulation RS generating unit    -   118 Channel coding unit    -   120 Data modulation unit    -   122 SC-FDMA modulation unit    -   200 Base station    -   202 Broadcast channel generating unit    -   204 OFDM signal generating unit    -   206 Radio-frame-number-and-subframe-number management unit    -   208 Uplink-scheduling-grant-signal-transmission-control-signal        generating unit    -   210 Memory for storing RS sequence numbers associated with        bandwidths in RS sequence groups    -   212 Cyclic-shift-number determining unit    -   214 Demodulation RS generating unit    -   216 Synchronization-detection/channel-estimation unit    -   218 Channel decoding unit    -   220 Coherent detection unit    -   222 Uplink-channel-condition estimation unit    -   224 Scheduler    -   300 Access gateway    -   400 Core network

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below with referenceto the accompanying drawings. Throughout the accompanying drawings, thesame reference numbers are used for parts having the same functions, andoverlapping descriptions of those parts are omitted.

A radio communication system 1000 including mobile stations and a basestation according to an embodiment of the present invention is describedbelow with reference to FIG. 5.

The radio communication system 1000 is based on, for example, EvolvedUTRA and UTRAN (also called Long Term Evolution or Super 3G). The radiocommunication system 1000 includes a base station (eNode B: eNB) 200 andmobile stations 100 _(n) (100 ₁, 100 ₂, 100 ₃, . . . 100 _(n); n is aninteger greater than 0). The base station 200 is connected to an uppernode such as an access gateway 300 and the access gateway 300 isconnected to a core network 400. The mobile stations 100 _(n) are in acell 50 and communicate with the base station 200 according to EvolvedUTRA and UTRAN.

The mobile stations 100 _(n) (100 ₁, 100 ₂, 100 ₃ . . . 100 _(n)) havethe same configuration and functions and are therefore called the mobilestation 100 _(n) or the mobile stations 100 _(n) in the descriptionsbelow unless otherwise mentioned. In the descriptions below, mobilestations are used as examples of user equipment (UE) communicating witha base station. However, user equipment communicating with a basestation may also include fixed terminals.

In the radio communication system 1000, orthogonal frequency divisionmultiplexing (OFDM) is used as the downlink radio access method andsingle-carrier frequency division multiple access (SC-FDMA) is used asthe uplink radio access method. As described above, OFDM is amulticarrier transmission scheme where a frequency band is divided intomultiple narrow frequency bands (subcarriers) and data are mapped to thesubcarriers for transmission. Meanwhile, SC-FDMA is a single-carriertransmission scheme where a frequency band is divided into multiplefrequency bands and the frequency bands are allocated to differentterminals in order to reduce interference between the terminals.

Communication channels used in Evolved UTRA and UTRAN are describedbelow.

For downlink, a physical downlink shared channel (PDSCH) shared by themobile stations 100 _(n) and a physical downlink control channel (PDCCH)are used. The physical downlink control channel is also called adownlink L1/L2 control channel. The physical downlink shared channel isused to transmit user data, i.e., a normal data signal.

For uplink, a physical uplink shared channel (PUSCH) shared by themobile stations 100 _(n) and a physical uplink control channel are used.The physical uplink shared channel is used to transmit user data, i.e.,a normal data signal. The physical uplink control channel is used totransmit a downlink channel quality indicator (CQI) used for schedulingand adaptive modulation and coding (AMC) of the physical downlink sharedchannel, and acknowledgement information for the physical downlinkshared channel. The acknowledgement information includes eitheracknowledgement (ACK) indicating normal reception of a transmittedsignal or negative acknowledgement (NACK) indicating abnormal receptionof a transmitted signal.

The physical uplink control channel may also be used to transmit, inaddition to the CQI and the acknowledgement information, a schedulingrequest for requesting allocation of resources of an uplink sharedchannel and a release request used in persistent scheduling. Here,allocation of resources of an uplink shared channel indicates a processwhere a base station reports to a mobile station by using the physicaldownlink control channel in a subframe that the mobile station isallowed to communicate using the uplink shared channel in a subsequentsubframe.

In the radio communication system 1000 of this embodiment, cell reuse isapplied but sequence hopping is not applied to sequences used forreference signals (RS) using a narrow bandwidth. Meanwhile, sequencesused for reference signals using a wide bandwidth are divided intogroups the number of which is the same as the total number of sequencesused for reference signals using a narrow bandwidth. The sequences ineach group are assigned to different subframes, i.e., sequence hoppingis performed using the sequences in each group. The total number ofsequences used for reference signals using a narrow bandwidth is equalto the cell reuse number. Hereafter, the groups of reference signalsequences associated with various bandwidths are called sequence groups.The number of the sequence groups is the same as the total number ofsequences used for reference signals using a narrow bandwidth. Thisapproach makes it possible to assign one of the sequence groups to acell and to assign another one of the sequence groups to a neighboringcell.

Thus, in this embodiment, sequence hopping is not employed for referencesignals using a narrow bandwidth, and sequence hopping is employed forreference signals using a wide bandwidth. As a result, the combinationof used sequences changes from subframe to subframe. This in turn makesit possible to prevent continuous occurrence of high interference andthereby to reduce the probability of successive packet errors.

Let us assume that the number of reference signal (RS) sequences for thenarrowest bandwidth W₁ is N₁ and the number of RS sequences for abandwidth W_(x) that is X times greater than W₁ is XN₁. The number ofsequences depends on the sequence length when CAZAC sequences are used,and the sequence length is in proportion to the bandwidth in E-UTRAwhere the transmission interval of the reference signal is constantregardless of the bandwidth. Therefore, in E-UTRA, the number ofsequences is proportional to the bandwidth.

In this embodiment, N₁ RS sequence groups are generated and a kth RSsequence group (k is an integer greater than or equal to 1 and less thanor equal to N₁) for a bandwidth W_(x) includes (W_(x)/W₁) sequences withsequence numbers k, k+N₁, . . . , and k+(W_(x)/W₁)N₁. With this method,an RS sequence does not belong to multiple RS sequence groups at thesame time. In each cell, one of the N₁ RS sequence groups is selectedand used. Sequence hopping is performed using only the RS sequencesassigned to the same bandwidth in the selected RS sequence group.

In the example shown in FIG. 6, one resource unit (RU) is the narrowestbandwidth and one RS sequence is provided for each RU in each of RSsequence groups 1-12. In FIG. 6, RS sequence [A, B] indicates the Bthsequence of RS sequences for a transmission bandwidth A.

For example, RS sequence group 1 is assigned to base station #1, RSsequence group 2 is assigned to base station #2, . . . , and RS sequencegroup 12 is assigned to base station #12. Thus, cell reuse where a setof sequence groups are cyclically assigned to different cells isemployed in this embodiment. When a narrow bandwidth, for example, 1 RU,is used, sequence hopping is not employed, but cell reuse where a set ofRS sequences are cyclically assigned to different cells is employed. Forexample, RS sequence 1 is assigned to base station #1, RS sequence 2 isassigned to base station #2, . . . , and RS sequence 12 is assigned tobase station #12. When 2 RUs are used, i.e., when the bandwidth of thereference signal is doubled, the number of sequences is also doubled.The doubled sequences are divided into groups the number of which equalsthe minimum number of sequences (12) such that each group includes apair of sequences to be assigned to a cell. The pair of sequences in agroup do not belong to other groups at the same time. Sequence hoppingis performed between the pair of sequences. For example, in a basestation to which RS sequence group 1 is assigned, sequence 1 is usedwhen the bandwidth is 1 RU and sequences 1 and 13 are used when thebandwidth is 2 RUs. Similarly, in a neighboring cell, sequence 2 is usedwhen the bandwidth is 1 RU and sequences 2 and 14 are used when thebandwidth is 2 RUs.

This approach makes it possible to prevent use of the same RS sequencein neighboring cells and also to randomize the interference by sequencehopping.

Next, the base station 200 of this embodiment is described withreference to FIG. 7.

The base station 200 of this embodiment includes a broadcast channelgenerating unit 202, an OFDM signal generating unit 204, aradio-frame-number-and-subframe-number management unit 206, anuplink-scheduling-grant-signal-transmission-control-signal generatingunit 208, a memory 210 for storing RS sequence numbers associated withbandwidths in RS sequence groups, a cyclic-shift-number determining unit212, a demodulation RS generating unit 214, asynchronization-detection/channel-estimation unit 216, a channeldecoding unit 218, a coherent detection unit 220, anuplink-channel-condition estimation unit 222, and a scheduler 224.

When cells are designed (base stations are installed), an uplink RSsequence group (number) is assigned to each cell. For each sequencegroup, sequence hopping patterns based on subframe numbers arepredetermined by specifications for bandwidths of two or more RUs.Accordingly, different RS sequence groups (numbers) are assigned toneighboring base stations. This approach makes it possible to randomizethe interference.

An RS sequence group number assigned to the base station 200 is input tothe broadcast channel generating unit 202 and the memory 210.

The broadcast channel generating unit 202 generates a broadcast channelincluding the input RS sequence group number and a system frame numberinput from the radio-frame-number-and-subframe-number management unit206 described later, and inputs the broadcast channel to the OFDM signalgenerating unit 204. The OFDM signal generating unit 204 generates anOFDM signal including the broadcast channel and inputs the OFDM signalto a radio transmitter. As a result, the assigned uplink RS sequencegroup is reported via the broadcast channel to all users in the cell.

Meanwhile, uplink channels received from the mobile stations 100 _(n)are input to the synchronization-detection/channel-estimation unit 216,the coherent detection unit 220, and the uplink-channel-conditionestimation unit 222.

The synchronization-detection/channel-estimation unit 216 performssynchronization detection for the input received signals to estimatetheir reception timings, performs channel estimation based ondemodulation reference signals input from the demodulation RS generatingunit 214 described later, and inputs the channel estimation results tothe coherent detection unit 220.

The coherent detection unit 220 performs coherent detection for thereceived signals based on the channel estimation results and allocatedfrequencies and bandwidths input from the scheduler 224 described later,and inputs the demodulated received signals to the channel decoding unit218. The channel decoding unit 218 decodes the demodulated receivedsignals and generates reproduced data signals corresponding to selecteduser numbers input from the scheduler 224. The generated reproduced datasignals are transmitted to a network.

The uplink-channel-condition estimation unit 222 estimates uplinkchannel conditions of respective users based on the input receivedsignals and inputs the estimated uplink channel conditions to thescheduler 224.

The scheduler 224 performs, for example, frequency scheduling based onthe input uplink channel conditions of the respective users and QoSinformation of the users such as requested data rates, buffer statuses,desired error rates, and delays. Then, the scheduler 224 inputsallocated frequencies and bandwidths to theuplink-scheduling-grant-signal-transmission-control-signal generatingunit 208, the memory 210, and the coherent detection unit 220, andinputs selected user numbers to theuplink-scheduling-grant-signal-transmission-control-signal generatingunit 208 and the channel decoding unit 218.

The cyclic-shift-number determining unit 212 determines cyclic shiftnumbers based on, for example, cooperative control signals transmittedbetween synchronized cells and inputs the cyclic shift numbers to theuplink-scheduling-grant-signal-transmission-control-signal generatingunit 208 and the demodulation RS generating unit 214. The cyclic shiftnumbers are associated with cyclic shift amounts. Assignment of thecyclic shift numbers is reported to the mobile stations 100 _(n) via,for example, a broadcast channel.

The radio-frame-number-and-subframe-number management unit 206 managesradio frame numbers and subframe numbers, inputs a system frame numberto the broadcast channel generating unit 202, and inputs the radio framenumbers and the subframe numbers to the memory 210.

The memory 210 stores the correspondence between RS sequence groupnumbers, bandwidths in respective RS sequence groups, and RS sequencenumbers as shown in FIG. 6. Also, the memory 210 selects RS sequencenumbers corresponding to the allocated bandwidths input from thescheduler 224 and inputs the selected RS sequence numbers to thedemodulation RS generating unit 214.

The demodulation RS generating unit 214 generates demodulation RSs basedon the RS sequence numbers input from the memory 210 and the cyclicshift numbers input from the cyclic-shift-number determining unit 212,and inputs the demodulation RSs to thesynchronization-detection/channel-estimation unit 216.

The uplink-scheduling-grant-signal-transmission-control-signalgenerating unit 208 generates a control signal(uplink-scheduling-grant-signal transmission control signal) includingthe allocated frequencies and bandwidths, the selected user numbers, andthe assigned cyclic shift numbers, and inputs the control signal to theOFDM signal generating unit 204. The OFDM signal generating unit 204generates an OFDM signal including the control signal and inputs theOFDM signal to the radio transmitter. As a result, the control signal istransmitted to scheduled users via a downlink control channel.

The OFDM signal generating unit 204 also generates an OFDM signalincluding downlink channels other than the broadcast channel and thecontrol channel such as a downlink reference signal, a data channel, anda paging channel, and inputs the OFDM signal to the radio transmitter.As a result, the downlink channels are transmitted to the users.

Next, the mobile station 100 _(n) of this embodiment is described withreference to FIG. 8.

The mobile station 100 _(n) of this embodiment includes an OFDM signaldemodulation unit 102, an uplink-scheduling-grant-signaldemodulation/decoding unit 104, a broadcast-channeldemodulation/decoding unit 106, an other-control-and-data-signalsdemodulation/decoding unit 108, a radio-frame-number-and-subframe-numbercounter 110, a cyclic-shift-amount determining unit 112, a memory 114for storing RS sequence numbers associated with bandwidths in RSsequence groups, a demodulation RS generating unit 116, a channel codingunit 118, a data modulation unit 120, and an SC-FDMA modulation unit122. The mobile station 100 _(n) decodes an uplink scheduling grantsignal and if a selected user number corresponding to the mobile station100 _(n) is included in the uplink scheduling grant signal, generatesand transmits a transmission signal.

A received signal from the base station 200 is input to the OFDM signaldemodulation unit 102. The OFDM signal demodulation unit 102 demodulatesthe received signal, inputs the uplink-scheduling-grant-signaltransmission control signal to the uplink-scheduling-grant-signaldemodulation/decoding unit 104, inputs the broadcast channel to thebroadcast-channel demodulation/decoding unit 106, and inputs control anddata signals other than the uplink-scheduling-grant-signal transmissioncontrol signal and the broadcast channel to theother-control-and-data-signals demodulation/decoding unit 108.

The broadcast-channel demodulation/decoding unit 106 demodulates anddecodes the input broadcast channel, inputs the RS sequence group numberto the memory 114, and inputs the system frame number to theradio-frame-number-and-subframe-number counter 110.

The radio-frame-number-and-subframe-number counter 110 counts radioframe numbers and subframe numbers and inputs the radio frame numbersand the subframe numbers to the memory 114.

The uplink-scheduling-grant-signal demodulation/decoding unit 104demodulates and decodes the input uplink scheduling grant signal, inputsthe assigned cyclic shift number to the cyclic-shift-amount determiningunit 112, inputs the allocated frequency to the SC-FDMA modulation unit122, and inputs the allocated bandwidth to the memory 114.

The memory 114 stores the correspondence between bandwidths inrespective RS sequence groups and RS sequence numbers as shown in FIG.6. The memory 114 also stores the correspondence between RS sequencenumbers and bandwidths in an RS sequence group assigned to the servingcell and reported by the base station 200. Further, the memory 114selects an RS sequence number based on the RS sequence group numberinput from the broadcast-channel demodulation/decoding unit 106 and theallocated bandwidth input from the uplink-scheduling-grant-signaldemodulation/decoding unit 104, and inputs the selected RS sequencenumber to the demodulation RS generating unit 116.

The cyclic-shift-amount determining unit 112 determines a cyclic shiftamount corresponding to the assigned cyclic shift number input from theuplink-scheduling-grant-signal demodulation/decoding unit 104, andinputs the determined cyclic shift amount to the demodulation RSgenerating unit 116.

The demodulation RS generating unit 116 generates a demodulation RSbased on the input RS sequence number and cyclic shift amount, andinputs the demodulation RS to the SC-FDMA modulation unit 122.

Meanwhile, the channel coding unit 118 performs channel coding on userdata. Then, the data modulation unit 120 performs data modulation on thechannel-coded user data and inputs the modulated user data to theSC-FDMA modulation unit 122.

The SC-FDMA modulation unit (DFT-spread OFDM) 122 modulates the inputdemodulation RS and the user data based on the allocated frequency andoutputs a transmission signal.

Next, an exemplary method of assigning reference signal sequences isdescribed with reference to FIG. 9.

In FIG. 9, it is assumed that RS sequence group 2 has already beenassigned to a base station 200 ₁ and an RS sequence group is to beassigned to a base station 200 ₂.

An RS sequence group different from RS sequence group 2 assigned to thebase station 200 ₁ is selected for the base station 200 ₂ to randomizethe interference (step S902). For example, RS sequence group 1 isselected for the base station 200 ₂.

Uplink RS sequence groups are assigned to respective cells when thecells are designed (base stations are installed) (step S904). For eachsequence group, sequence hopping patterns based on subframe numbers arepredetermined by specifications.

The base station 200 ₂ reports the assigned uplink RS sequence group viaa broadcast channel to all users in the cell (step S906).

The base station 200 ₂ also reports cyclic shift amounts together withbandwidth allocation information to scheduled users via a downlinkcontrol channel (step S908).

Each scheduled mobile station (terminal) 100 _(n) determines an RSsequence used for the allocated bandwidth based on a table (FIG. 6)corresponding to the reported RS sequence group and an allocatedsubframe number, shifts the determined RS sequence by the cyclic shiftamount reported via the control channel, and transmits an uplink channelincluding the shifted RS sequence (step S910).

In this embodiment, assigned reference signal sequence numbers arereported to terminals by reporting a sequence group number assigned tothe serving cell to the terminal. Alternatively, cell IDs may beassociated with sequence group numbers in advance, or reference signalsequence numbers may be reported from the serving cell to scheduledterminals together with control information indicating a schedulinggrant (uplink-scheduling-grant-signal transmission control signal).Associating cell IDs with sequence group numbers in advance eliminatesthe need to report the sequence group numbers to terminals.

In this embodiment, hopping patterns used for each sequence group arepredetermined, i.e., sequence hopping patterns based on subframe numbersare predetermined by specifications for each sequence group.Alternatively, hopping patterns may be reported to terminals from thebase station. For example, hopping patterns may be reported via abroadcast channel, or hopping patterns determined by dynamic hoppingcontrol may be reported.

Also in this embodiment, the cyclic shift amount is reported as a partof format information for the uplink reference signal. Morespecifically, cyclic shift amounts associated with cyclic shift numbersare predetermined by the base station taking into account the cellradius and the delay spread and reported to terminals via a broadcastchannel; and assigned cyclic shift amounts are reported dynamically tothe terminals together with the scheduling grant. For example, thecorrespondence between cyclic shift numbers and cyclic shift amounts ispredetermined and reported via broadcast information, and after cyclicshift amounts are assigned, the corresponding cyclic shift numbers arereported to the terminals.

Also, orthogonal covering sequences may be reported as the formatinformation for uplink reference signals. Orthogonal covering may beused for orthogonalization of multiple antennas of a MIMO user. When auser is requested to use MIMO, the user orthogonalizes reference signalsfrom multiple antennas by orthogonal covering without additionalsignaling. With orthogonal covering, the same CAZAC sequence and thesame cyclic shift amount are used for two reference signals in asubframe. The orthogonal covering may also be used for orthogonalizationof users. In this case, orthogonal covering sequences are reporteddynamically together with the scheduling grant.

Next, selective use of orthogonalization and interference randomizationis described with reference to FIG. 10.

In FIG. 10, it is assumed that uplink transmission timings of basestation #1, base station #2, and base station #3 are asynchronous. Eachbase station covers two sectors and transmission timings of sectorsbelonging to the same base station can be synchronized.

The sectors can be orthogonalized by using different cyclic shiftamounts even if the same RS sequence group is assigned. For example,although the same RS sequence group 1 is assigned to sectors 1 and 2 ofbase station #1, sectors 1 and 2 can be orthogonalized because differentcyclic shift amounts are used. Similarly, although the same RS sequencegroup is assigned to sectors 1 and 2 of base station #2, sectors 1 and 2can be orthogonalized because different cyclic shift amounts are used.

Also, the interference between base stations #1 and #2 can be randomizedby assigning different RS sequence groups to base stations #1 and #2. Inthis example, the interference is randomized by assigning RS sequencegroup 1 to base station #1 and assigning RS sequence group 2 to basestation #2.

In the case of a MIMO terminal, multiple antennas can be orthogonalizedby orthogonal covering or by cyclic shifting.

Also, different RS sequence groups may be assigned even to synchronizedcells (e.g., sectors 1 and of base station #3) to achieve interferencerandomization.

As described above, in this example, it is assumed that the basestations are not synchronized. In a case where base stations aresynchronized, the same RS sequence group and different cyclic shiftamounts may be assigned to the base stations to orthogonalize usersusing the same frequency band with the same bandwidth.

Next, a process in the radio communication system 1000 of thisembodiment is described with reference to FIG. 11.

Here, it is assumed that an RS sequence group is assigned in advance tothe base station 200. In each user terminal, information regarding allRS sequence groups available in the system (correspondence betweensequence groups and RS sequences and correspondence between RS sequencesand subframe/radio frame numbers (hopping patterns)) is stored inadvance. User terminal 2 employs multi-antenna MIMO.

The base station 200 transmits system information via a broadcastchannel (step S1102). For example, the base station 200 broadcasts an RSsequence group number and a system frame number.

The base station 200 also transmits a paging channel (step S1104). Forexample, the base station 200 transmits a paging channel to page userterminals 1 and 2, i.e., when there are incoming calls for the userterminals 1 and 2.

In response to the paging channel, the user terminals 1 and 2 transmitrandom access channels for initial access (steps S1106 and S1108), Thebase station 200 and the user terminals 1 and 2 exchange controlchannels (steps S1110 and S1112). As a result, radio links areestablished between the base station 200 and the user terminals 1 and 2.At this stage, the base station 200 recognizes that the user terminal 2is a MIMO terminal.

After the above steps are conducted, packet communications based ondownlink scheduling are enabled. The user terminals 1 and 2 transmitwideband sounding reference signals for CQI measurement at predeterminedintervals.

The base station 200 performs scheduling for the user terminal 1 (stepS1114).

The base station 200 transmits an uplink scheduling grant signal to theuser terminal 1 (step S1116). The uplink scheduling grant signalincludes a selected user number, an allocated frequency band for uplink,and an assigned cyclic shift number.

The user terminal 1 determines an assigned RS sequence based on theallocated bandwidth, the subframe number, and the radio frame numberused for transmission. The user terminal 1 shifts the RS sequence by acyclic shift amount corresponding to the cyclic shift number reported bythe uplink scheduling grant signal and transmits an uplink channelincluding the shifted RS sequence (step S1118).

The base station 200 performs scheduling for the user terminal 2 (stepS1120).

The base station 200 transmits an uplink scheduling grant signal to theuser terminal 2 (step S1122). The uplink scheduling grant signalincludes a selected user number, an allocated frequency band for uplink,and an assigned cyclic shift number.

The user terminal 2 determines an assigned RS sequence based on theallocated bandwidth, the subframe number, and the radio frame numberused for transmission. The user terminal 2, which is a MIMO terminal,shifts the RS sequence by a cyclic shift amount corresponding to thecyclic shift number reported by the uplink scheduling grant signal andtransmits the shifted RS sequence from respective antennas 1 and 2 aftermultiplying the RS sequence by the corresponding orthogonal coveringsequence predetermined by the system (step S1124).

The descriptions and drawings in the above embodiments should not beconstrued to be limiting the present invention. A person skilled in theart may think of variations of the above embodiments from thedescriptions.

In other words, the present invention may also include variousembodiments not disclosed above. Therefore, the technical scope of thepresent invention should be determined based on proper understanding ofthe claims with reference to the above descriptions.

Although the present invention is described above in differentembodiments, the distinctions between the embodiments are not essentialfor the present invention, and the embodiments may be used individuallyor in combination. Although specific values are used in the abovedescriptions to facilitate the understanding of the present invention,the values are just examples and different values may also be usedunless otherwise mentioned.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention. Although functionalblock diagrams are used to describe apparatuses in the aboveembodiments, the apparatuses may be implemented by hardware, software,or a combination of them.

The present international application claims priority from JapanesePatent Application No. 2007-073728 filed on Mar. 20, 2007, the entirecontents of which are hereby incorporated herein by reference.

1. A base station for communicating with a mobile station transmittingan uplink signal according to a single-carrier scheme, wherein one ofsequence groups each specifying reference signal sequences forrespective radio resource bandwidths is assigned to the base station, adifferent one of the sequence groups is assigned to a neighboring cell,and the mobile station transmits the uplink signal including one of thereference signal sequences specified by the one of the sequence groupsassigned to the base station, the base station comprising: a schedulerconfigured to allocate radio resources such that one or more resourceunits are allocated to the mobile station for communication; a reportingunit configured to report the allocated radio resources and a cyclicshift amount to the mobile station; and a demodulating unit configuredto demodulate the uplink signal received from the mobile station basedon the one of the reference signal sequences corresponding to one of theradio resource bandwidths and the cyclic shift amount, wherein cellreuse is applied to the reference signal sequences to be transmittedusing one resource unit; and sequence hopping where different ones ofthe reference signal sequences are assigned to consecutive subframes isapplied to the reference signal sequences to be transmitted using abandwidth greater than one resource unit.
 2. The base station as claimedin claim 1, further comprising: a broadcasting unit configured tobroadcast the one of the sequence groups assigned to the base station tothe mobile station.
 3. The base station as claimed in claim 1, whereinone of the reference signal sequences to be transmitted using oneresource unit is statically assigned to the base station; the referencesignal sequences to be transmitted using the bandwidth greater than oneresource unit are divided into sequence groups the number of whichequals a total number of the reference signal sequences to betransmitted using one resource unit; and the reference signal sequencesin each one of the sequence groups are assigned to different subframes.4. The base station as claimed in claim 3, wherein when one resourceunit is a minimum bandwidth W₁, a number of the reference signalsequences using the minimum bandwidth W₁ is N₁, and a number of thereference signal sequences using a bandwidth W_(x) that is X timesgreater than the minimum bandwidth W₁ is XN₁, N₁ sequence groups aregenerated; and a kth sequence group (k is an integer greater than orequal to 1 and less than or equal to N₁) includes (W_(x)/W₁) referencesignal sequences with sequence numbers k, k+N₁, . . . , andk+(W_(x)/W₁)N₁ for the bandwidth W_(x).
 5. The base station as claimedin claim 4, wherein one of the N₁ sequence groups is assigned to thebase station.
 6. The base station as claimed in claim 4, wherein when aradio resource bandwidth allocated to the mobile station is W_(x), thesequence hopping is performed using the (W_(x)/W₁) reference signalsequences with sequence numbers k, k+N₁, . . . , and k+(W_(x)/W₁)N₁. 7.The base station as claimed in claim 6, wherein one or more hoppingpatterns are predetermined for each of the N₁ sequence groups.
 8. Thebase station as claimed in claim 1, wherein the reference signalsequences are CAZAC sequences.
 9. A mobile station transmitting anuplink signal according to a single-carrier scheme, comprising: astorage unit configured to store sequence groups each specifyingreference signal sequences for respective radio resource bandwidths; atransmitting unit configured to determine one of the reference signalsequences corresponding to one of the radio resource bandwidths based onradio resources allocated by a base station, to shift the determined oneof the reference signal sequences by a cyclic shift amount assigned bythe base station, and to transmit the uplink signal including theshifted one of the reference signal sequences, wherein cell reuse isapplied to the reference signal sequences to be transmitted using oneresource unit; and sequence hopping where different ones of thereference signal sequences are assigned to consecutive subframes isapplied to the reference signal sequences to be transmitted using abandwidth greater than one resource unit.
 10. The mobile station asclaimed in claim 9, wherein one of the reference signal sequences to betransmitted using one resource unit is statically assigned to the basestation; the reference signal sequences to be transmitted using thebandwidth greater than one resource unit are divided into sequencegroups the number of which equals a total number of the reference signalsequences to be transmitted using one resource unit; and the referencesignal sequences in each one of the sequence groups are assigned todifferent subframes.
 11. The mobile station as claimed in claim 10,wherein one or more hopping patterns are predetermined for each of thesequence groups.
 12. The mobile station as claimed in claim 9, whereinthe mobile station includes multiple antennas; and the transmitting unitis configured to use the same one of the reference signal sequences fortwo reference signals in a subframe.
 13. The mobile station as claimedin claim 9, wherein the reference signal sequences are CAZAC sequences.14. A radio communication system, comprising: a mobile stationconfigured to transmit an uplink signal according to a single-carrierscheme; and a base station configured to communicate with the mobilestation, wherein one of sequence groups each specifying reference signalsequences for respective radio resource bandwidths is assigned to thebase station; a different one of the sequence groups is assigned to aneighboring cell; the mobile station is configured to transmit theuplink signal including one of the reference signal sequences specifiedby one of the sequence groups assigned to the base station; the basestation includes a scheduler configured to allocate radio resources suchthat one or more resource units are allocated to the mobile station forcommunication, a reporting unit configured to report the allocated radioresources and a cyclic shift amount to the mobile station, and ademodulating unit configured to demodulate the uplink signal receivedfrom the mobile station based on the one of the reference signalsequences corresponding to one of the radio resource bandwidths and thecyclic shift amount; the mobile station includes a storage unitconfigured to store the sequence groups, and a transmitting unitconfigured to determine the one of the reference signal sequencescorresponding to the one of the radio resource bandwidths based on theradio resources allocated by the base station, to shift the determinedone of the reference signal sequences by the cyclic shift amountreported by the base station, and to transmit the uplink signalincluding the shifted one of the reference signal sequences; cell reuseis applied to the reference signal sequences to be transmitted using oneresource unit; and sequence hopping where different ones of thereference signal sequences are assigned to consecutive subframes isapplied to the reference signal sequences to be transmitted using abandwidth greater than one resource unit.
 15. A communication controlmethod used in a radio communication system including a mobile stationtransmitting an uplink signal according to a single-carrier scheme and abase station communicating with the mobile station, wherein one ofsequence groups each specifying reference signal sequences forrespective radio resource bandwidths is assigned to the base station anda different one of the sequence groups is assigned to a neighboringcell, the method comprising: a radio resource allocation step, performedby the base station, of allocating radio resources such that one or moreresource units are allocated to the mobile station for communication; areporting step, performed by the base station, of reporting theallocated radio resources and a cyclic shift amount to the mobilestation; a transmitting step, performed by the mobile station, oftransmitting the uplink signal based on the radio resources and thecyclic shift amount reported by the base station; and a demodulatingstep, performed by the base station, of demodulating the uplink signalreceived from the mobile station based on one of the reference signalsequences corresponding to one of the radio resource bandwidths and thecyclic shift amount, wherein cell reuse is applied to the referencesignal sequences to be transmitted using one resource unit; and sequencehopping where different ones of the reference signal sequences areassigned to consecutive subframes is applied to the reference signalsequences to be transmitted using a bandwidth greater than one resourceunit.
 16. The mobile station as claimed in claim 10, wherein the mobilestation includes multiple antennas; and the transmitting unit isconfigured to use the same one of the reference signal sequences for tworeference signals in a subframe.
 17. The mobile station as claimed inclaim 11, wherein the mobile station includes multiple antennas; and thetransmitting unit is configured to use the same one of the referencesignal sequences for two reference signals in a subframe.